Long-term dynamics of the characteristics of tree cavities used for nesting by vertebrates

Abstract

Many cavity-nesting vertebrates rely on tree cavities for nesting, and these cavities are often a limiting resource. The availability of tree cavities depends on rates of formation, persistence, and degradation or change in characteristics (e.g., entrance area, chamber volume, level of decay). Understanding how cavity characteristics change over time is key to understanding the value of cavities for wildlife as they age. We measured physical characteristics of 960 tree cavities for up to 17years (2490 measures of cavity dimensions, involving 23 species of cavity-using vertebrates) to estimate rates of change in cavities in relation to their age, whether they were in living versus dead trees, and their formation agent. We also analyzed the effects of cavity expansion by renovators. Overall, we found that cavity depth increased by 0.35cmyr-1 (1.7%), horizontal depth increased by 0.09cmyr-1 (0.8%), cavity entrance diameter increased by 0.06cmyr-1 (1.2%), and the decay class of the cavity tree advanced by one category every 12.5years. Cavities in living trees increased in depth by 0.84cmyr-1 versus 0.14cmyr-1 in dead trees; both showed relatively small increases in entrance diameter, with a non-significant trend toward greater increase of entrance diameter in dead trees. Cavities excavated by northern flickers [Colaptes auratus] maintained relatively constant dimensions compared with those created by other excavators, although cavity trees selected by northern flickers decayed 3.3 times more rapidly than those selected by strong excavators (hairy woodpeckers [Picoides villosus], American three-toed woodpeckers [Picoides dorsalis], pileated woodpeckers [Dryocopus pileatus], and red-naped sapsuckers [Sphyrapicus nuchalis]). Northern flickers were the most frequent renovators with 13.5% of cavities they used renovated from cavities formed by other excavators or decay. Renovations by northern flickers increased cavity depth by a mean of 16.5±11.8cm. We show that cavities in living trees significantly increase in cavity volume as they age and likely increase in quality over time, while cavities in dead trees maintain relatively constant dimensions. Thus, retention of living trees as "wildlife trees" is at least as important as retention of dead trees (snags) to support a diverse community of cavity-nesting birds and mammals.